Semiconductor light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs) are of great interest as highly efficient and robust light sources. Efforts to extend operation of such devices into the shorter wavelength of the visible spectrum (e.g., from green to violet) have led to use of group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen (i.e., III-nitride materials) and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus (i.e., III-phosphide materials). In a typical LED, a p-type layer of a semiconductor material is in contact with a n-type layer of the semiconductor material to form an LED chip. Electrodes are attached to each of the layers, and the assembly is typically encapsulated in a substantially transparent material to complete the LED device. When a voltage is applied across the two layers of semiconductor material, the p-n junction of the LED emits light.
A disadvantage associated with current LEDs is that the encapsulating materials typically have a lower refractive index than that of the LED chip. The refractive index of a LED chip is typically between 2.4 and 3.6, while the refractive index of epoxy or plastic encapsulants is typically between 1.4 and 1.6. In order for light to escape the LED device, the light emitted from the LED chip must cross the boundary between the LED chip and the encapsulating material. If the angle of incidence of the light to the boundary between materials is shallower than the critical angle, the light will be reflected back into the LED, due to the phenomenon of total internal reflection, thus diminishing the amount of light that can escape the LED device and diminishing the efficiency of the device. The critical angle is given by the equation: θc=sin−1 (n2/n1) wherein n1 is the refractive index of the LED chip and n2 is the refractive index of the encapsulating material. Therefore, an encapsulating material having a low refractive index n2 will have a decreased critical angle θc compared to an encapsulating material having a higher refractive index n2, thereby increasing the amount of light lost to total internal reflection.
In addition, when light crosses a boundary between media of two different refractive indices, a portion of the light is reflected back from the interface between the two media due to the difference in refractive indices. This phenomenon is known as Fresnel reflection or Fresnel loss. The amount of Fresnel loss is greater with a greater difference in refractive index between the two media. Although the amount of light loss due to Fresnel reflection is less than the amount of light loss due to total internal reflection, the Fresnel loss accounts for a significant loss of light produced by the LED device. If the refractive index of the encapsulating material could be increased, thereby decreasing the difference between the refractive indices of the LED chip and the encapsulating material, Fresnel loss would be reduced.
Attempts have been made to produce encapsulating materials for LEDs having higher refractive indices than current epoxies and other polymers. For example, chalcogenide glasses have been investigated as encapsulants for LEDs. However, difficulties in processing such glasses make them unsuitable for mass production. The incorporation of nanoparticles of high refractive index such as titanium dioxide into polymeric host materials has been reported in U.S. Pat. No. 5,777,433. However, the titanium dioxide particles tend to aggregate and/or agglomerate during incorporation into the host material, requiring treatment with an anti-flocculant coating prior to mixing with the host material. In addition, the requirement for a low refractive index host material and the limitation on the amount of high refractive index nanoparticles that can be incorporated into the host material without subsequent aggregation and/or agglomeration limits the utility of such an approach.
Further, as LEDs are being developed to generate higher light output, the increased power necessary to generate such higher light outputs result in higher operating temperatures for LED devices. Increasing the amount of extractable light generated by LED devices would allow for a reduction in power required for a given light output, with attendant reduction in operating temperatures.
The invention provides an encapsulating material and a light emitting device comprising the same. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.